Lipopolyamines: novel antiendotoxin compounds that reduce mortality in experimental sepsis caused by gram-negative bacteria

An infection-microenvironment-targeted and responsive peptide-drug nanosystem for sepsis emergency by suppressing infection and inflammation

Sepsis is a life-threatening emergency that causes millions of deaths every year due to severe infection and inflammation. Nevertheless, current therapeutic regimens are inadequate to promptly address the vast diversity of potential pathogens. Omiganan, an antimicrobial peptide, has shown promise for neutralizing endotoxins and eliminating diverse pathogens. However, its clinical application is hindered by safety and stability concerns. Herein, we present a nanoscale drug delivery system (Omi-hyd-Dex@HA NPs) that selectively targets infectious microenvironments (IMEs) and responds to specific stimuli for efficient intervention in sepsis. The system consists of omiganan-dexamethasone conjugates linked by hydrazone bonds which self-assemble into nanoparticles coated with a hyaluronic acid (HA). The HA coating not only facilitates IMEs-targeting through interaction with intercellular-adhesion-molecule-1 on inflamed endotheliocytes, but also improves the biosafety of the nanosystem and enhances drug accumulation in primary infection sites triggered by hyaluronidase. The nanoparticles release dual drugs in IMEs through pH-sensitive cleavage of hydrazone bonds to eradicate pathogens and suppress inflammation. In multiple tissue infection and sepsis animal models, Omi-hyd-Dex@HA NPs exhibited rapid source control and comprehensive inflammation reduction, thereby preventing subsequent fatal complications and significantly improving survival outcomes. The bio-responsive and self-delivering nanosystem offers a promising strategy for systemic sepsis treatment in emergencies.

Synergy by Perturbing the Gram-Negative Outer Membrane: Opening the Door for Gram-Positive Specific Antibiotics

New approaches to target antibacterial agents toward Gram-negative bacteria are key, given the rise of antibiotic resistance. Since the discovery of polymyxin B nonapeptide as a potent Gram-negative outer membrane (OM)-permeabilizing synergist in the early 1980s, a vast amount of literature on such synergists has been published. This Review addresses a range of peptide-based and small organic compounds that disrupt the OM to elicit a synergistic effect with antibiotics that are otherwise inactive toward Gram-negative bacteria, with synergy defined as a fractional inhibitory concentration index (FICI) of <0.5. Another requirement for the inclusion of the synergists here covered is their potentiation of a specific set of clinically used antibiotics: erythromycin, rifampicin, novobiocin, or vancomycin. In addition, we have focused on those synergists with reported activity against Gram-negative members of the ESKAPE family of pathogens namely, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and/or Acinetobacter baumannii. In cases where the FICI values were not directly reported in the primary literature but could be calculated from the published data, we have done so, allowing for more direct comparison of potency with other synergists. We also address the hemolytic activity of the various OM-disrupting synergists reported in the literature, an effect that is often downplayed but is of key importance in assessing the selectivity of such compounds for Gram-negative bacteria.

Adsorption-Based Detoxification of Endotoxins by Porous Flexible Organic Frameworks

Bacterial lipopolysaccharides (LPS, endotoxins) cause sepsis that is responsible for a huge amount of mortality globally. However, their neutralization or detoxification remains an unmet medical need. We envisaged that cationic organic frameworks with persistent hydrophobic porosity may adsorb and thus neutralize LPS through a combination of cooperative ion-pairing electrostatic attraction and hydrophobicity. We here report the preparation of two water-soluble flexible organic frameworks (FOF-1 and FOF-2) from tetratopic and ditopic precursors through quantitative formation of hydrazone bonds at room temperature. The two FOFs are revealed to possess hydrodynamic diameters, which range from 20 to 120 nm, depending on the concentrations. Dynamic light scattering and isothermal titration calorimetric and chromogenic limulus amebocyte lysate experiments indicate that both frameworks are able to adsorb and thus reduce the concentration of free LPS molecules in aqueous solution, whereas cytokine inhibition experiments with RAW264.7 support that this adsorption can significantly decrease the toxicity of LPS. In vivo experiments with mice (five males per group) show that the injection of FOF-1 at a dose of 0.6 mg/kg realizes the survival of all of the mice administrated with LPS of the d-galactosamine (d-Gal)-sensitized absolute lethal dose (LD₁₀₀, 0.05 mg/kg), whereas its maximum tolerated dose for mice is determined to be 10 mg/kg. These findings provide a new promising sequestration strategy for the development of porous agents for the neutralization of LPS.

An update on endotoxin neutralization strategies in Gram-negative bacterial infections

INTRODUCTION

Gram-negative bacterial infections represent still a severe problem of human health care, regarding the increase in multi-resistance against classical antibiotics and the lack of newly developed antimicrobials. For the fight against these germs, anti-infective agents must overcome and/or bind to the Gram-negative outer membrane consisting of a lipopolysaccharide (LPS, endotoxin) outer leaflet and an inner leaflet from phospholipids, with additional peripheral or integral membrane proteins (OMP's).

AREAS COVERED

The current article reviews data of existing therapeutic options and summarizes newer approaches for targeting and neutralizing endotoxins, ranging from in vitro over in vivo animal data to clinical applications by using databases such as Medline.

EXPERT OPINION

Conventional antibiotic treatment of the bacteria leads to their killing, but not necessary LPS neutralization, which may be a severe problem in particular for the systemic pathway. This is the reason why there is an increasing number of therapeutic approaches, which - besides combating whole bacteria - at the same time try to neutralize endotoxin within or outside the bacterial cells mainly responsible for the high inflammation induction in Gram-negative species.

Recent advances in nanomedicine for sepsis treatment

Sepsis and septic shock are life-threating conditions, which form a continuum of the body's response to overwhelming infection. The current treatment consists of fluid and metabolic resuscitation, hemodynamic and end-organ support, and timely initiation of antibiotics. However, these measures may be ineffective and the sepsis-related mortality toll remains substantial; therefore, an urgent need exists for new therapies. Recently, several nanoparticle (NP) systems have shown excellent protective effects against sepsis in preclinical models, suggesting a potential utility in the management of sepsis and septic shock. These NPs serve as antibacterial agents, provide platforms to immobilize endotoxin adsorbents, interact with inflammatory cells to restore homeostasis and detect biomarkers of sepsis for timely diagnosis. This review discusses the recent developments in NP-based approaches for the treatment of sepsis.

Enhancing Whole Phage Therapy and Their Derived Antimicrobial Enzymes through Complex Formulation

The resurgence of research into phage biology and therapy is, in part, due to the increasing need for novel agents to treat multidrug-resistant infections. Despite a long clinical history in Eastern Europe and initial success within the food industry, commercialized phage products have yet to enter other sectors. This relative lack of success is, in part, due to the inherent biological limitations of whole phages. These include (but are not limited to) reaching target sites at sufficiently high concentrations to establish an infection which produces enough progeny phages to reduce the bacterial population in a clinically meaningful manner and the limited host range of some phages. Conversely, parallels can be drawn between antimicrobial enzymes derived from phages and conventional antibiotics. In the current article the biological limitations of whole phage-based therapeutics and their derived antimicrobial enzymes will be discussed. In addition, the ability of more complex formulations to address these issues, in the context of medical and non-medical applications, will also be included.

Polyethylenimine-dermatan sulfate complex, a bioactive biomaterial with unique toxicity to CD146-positive cancer cells

We report unique bioactivity of a polycation-polyanion complex with potential utility for cancer therapy. A complex of disulfide-crosslinked polyethyleneimine (CLPEI), a polycation used for gene complexation, and dermatan sulfate (DS), an anionic polysaccharide to shield excessive cationic charge of the former, has toxicity to a specific group of cancer cell lines, including B16-F10 murine melanoma, A375SM human melanoma, and PC-3 human prostate cancer cells. These CLPEI-DS-sensitive cells express CD146, which binds to the complex via interaction with DS. There is a positive correlation between toxicity and intracellular level of CLPEI, indicating that the CLPEI-DS-sensitivity is attributable to the increased cellular uptake of CLPEI mediated by the DS-CD146 interactions. In vitro studies show that CLPEI-DS complex causes G₀/G₁ phase arrest and apoptotic cell death. In syngeneic and allograft models of B16-F10 melanoma, CLPEI-DS complex administered with a sub-toxic level of doxorubicin potentiates the chemotherapeutic effect of the drug by loosening tumor tissues. Given the unique toxicity, CLPEI-DS complex may be a useful carrier of gene or chemotherapeutics for the therapy of CD146-positive cancers.

Branched-chain and dendritic lipids for nanoparticles

Lipid nanoparticles (LNPs) for drug-delivery applications are largely derived from natural lipids. Synthetic lipids, particularly those incorporating branched hydrocarbons and hyper-branched hydrocarbon architectures, may afford enhanced lipophilicity with enhanced fluidity and thereby lead to LNP stabilization. Hydrocarbon anchors based on serinol diesters were prepared from linear Cn (n = 14, 16, 18) and branched (n = 16) acids with Boc-protected serinol. These diesters were further dimerized on an iminodiacetamide backbone to provide eight branched-chain and dendritic lipid anchors. Derivatization of these core structures provided eight PEG-lipids and seven thiopurine linked lipid–drug conjugates. LNPs were prepared by microfluidic mixing from mixed lipids in ethanol diluted into aqueous media. The lipid–drug conjugates incorporated 5 mol% of a phosphocholine and 5 mol% of a commercial PEG-lipid to form LNPs with a thiopurine drug loading of 15 wt%. The PEG–lipids prepared were formulated at 1.5 mol% as a surface stabilizer to LNPs containing dsDNA lipoplexes. The stability of the LNPs was assessed under different storage conditions through monitoring of particle size. For both LNPs from lipid–thiopurine conjugates and the PEG-lipid systems, there is strong preliminary evidence that hydrocarbon branching results in LNP stabilization. Four of the lipid–drug conjugate formulations were stable to cell culture conditions (10% serum, 37 °C) and the toxicity of these LNPs was assessed in two cell lines relative to the free thiopurines in the medium. The observed toxicity is consistent with cellular uptake of the LNPs and reductive release of the cargo thiopurine within the cell.

In vitro flow-through assay for rapid detection of endotoxin in human sera: A proof-of-concept

An increase in endotoxin concentration in the bloodstream can trigger activation of innate immune response leading to septic shock. There is currently no method available for rapid endotoxin detection at a patient's bedside. We demonstrate a simple, portable and cost-effective strategy to measure endotoxin levels in human serum within 5min using a flow-through assay. A drop of serum containing LPS was spotted on an endotoxin-affinity membrane placed over high-wicking absorbent pads. Subsequent addition of polymyxin B sulfate drug-conjugated gold nanoparticles allowed concentration-dependent visualization of spots by the naked eye in the clinically-relevant range of 10pg/mL to 10ng/mL. The results were quantified using a concentration-calibrated color chart and the assay performance was tested with archival plasma samples of 18 known septicemia patients. The results showed a reasonably good correlation with the patients' hematological data. This proof-of-concept study puts forth an interesting alternative for early septicemia diagnosis in future.

Endotoxin Entrapment on Glass via C-18 Self-Assembled Monolayers and Rapid Detection Using Drug-Nanoparticle Bioconjugate Probes

Bloodstream bacterial infections are known to illicit a systemic immune response that can lead to multiorgan failure and septic shock. The current endotoxin identification techniques in serum are expensive and elaborate requiring bulky benchtop instrumentation. We demonstrate a new route for endotoxin detection in which lipopolysaccharides (LPS) in solution are entrapped using C-18 silane-functionalized glass slides and tagged with polymyxin B sulfate (PMB) drug-conjugated gold nanoparticles. The signal from the particles is further amplified via the silver reduction approach to yield concentration-dependent colorimetric spots visible to the bare eye. The method is rapid, reliable, and cost-effective and fulfills an urgent unmet need in the healthcare industry for early septicemia diagnosis.

The role of polar and facial amphipathic character in determining lipopolysaccharide-binding properties in synthetic cationic peptides

Two series of peptides, designated K and NK were synthesized and tested for lipid A binding and neutralizing properties. K2, which has an 11-residue amphiphilic core, and a branched N-terminus bearing two branched lysinyl residues does not bind lipid A, while NK2, also with an 11-residue amphiphilic core comprised entirely of non-ionizable residues, and a similarly branched, cationic N-terminus, binds lipid A very weakly. Both peptides do not inhibit lipopolysaccharide (LPS) activity in the Limulus assay, nor do they inhibit LPS-induced TNF-α and NO production in J774 cells. These results are entirely unlike a homologous peptide with an exclusively hydrophobic core whose LPS-binding and neutralizing properties are very similar to that of polymyxin B [David SA, Awasthi SK, Wiese A et al. Characterization of the interactions of a polycationic, amphiphilic, terminally branched oligopeptide with lipid A and lipopolysaccharide from the deep rough mutant of Salmonella minnesota . J Endotoxin Res 1996; 3: 369—379]. These data suggest that a clear segregation of charged and apolar domains is crucial in molecules designed for purposes of LPS sequestration and that head-tail (polar) orientation of the cationic/hydrophobic regions is preferable to molecules with mixed or facial cationic/amphipathic character.

Review: D-Galactosamine lethality model: scope and limitations

D-Galactosamine (D-galN) is well established as sensitizing mice and other animals to the lethal effects of TNF, specifically, and by several orders of magnitude. Protection by anti-TNF neutralizing antibody is complete, as is (metabolically-based) protection by uridine. Sensitization occurs regardless of the origin of the released TNF, whether it is released from macrophages and/or T-cells. The same is true for the challenging agent which leads to the release of TNF, whether it is endotoxin, a superantigen, lipoprotein, bacterial DNA, or bacteria, either killed or proliferating. Most studies have utilized endotoxin as the challenging agent, and more than 70 agents have been reported to confer protection against LPS and/or TNF challenge in the model. The model has provided new insight regarding modes of protection, including from dexamethasone, which protects against challenge from LPS but not from challenge by TNF. The D-galN lethality model has also been used to test for synergistic behavior between different bacterial components, and to test for lethality when only small amounts of the challenging agent are available (lipid A chemistry).

Invited review: Mechanisms of endotoxin neutralization by synthetic cationic compounds

A basic challenge in the treatment of septic patients in critical care units is the release of bacterial pathogenicity factors such as lipopolysaccharide (LPS, endotoxin) from the cell envelope of Gram-negative bacteria due to killing by antibiotics. LPS aggregates may interact with serum and membrane proteins such as LBP (lipopolysaccharide-binding protein) and CD14 leading to the observed strong reaction of the immune system. Thus, an effective treatment of patients infected by Gram-negative bacteria must comprise beside bacterial killing the neutralization of endotoxins. Here, data are summarized for synthetic compounds indicating the stepwise development to very effective LPS-neutralizing agents. These data include synthetic peptides, based on the endotoxin-binding domains of natural binding proteins such as lactoferrin, Limulus anti-LPS factor, NK-lysin, and cathelicidins or based on LPS sequestering polyamines. Many of these compounds could be shown to act not only in vitro, but also in vivo (e.g . in animal models of sepsis), and might be useful in future clinical trials and in sepsis therapy.

Zwitterionic chitosan for the systemic treatment of sepsis

Severe sepsis and septic shock are life-threatening conditions, with Gram-negative organisms responsible for most sepsis mortality. Systemic administration of compounds that block the action of lipopolysaccharide (LPS), a constituent of the Gram-negative outer cell membrane, is hampered by their hydrophobicity and cationic charge, the very properties responsible for their interactions with LPS. We hypothesize that a chitosan derivative zwitterionic chitosan (ZWC), previously shown to suppress the production of pro-inflammatory cellular mediators in LPS-challenged macrophages, will have protective effects in an animal model of sepsis induced by systemic injection of LPS. In this study, we evaluate whether ZWC attenuates the fatal effect of LPS in C57BL/6 mice and investigate the mechanism by which ZWC counteracts the LPS effect using a PMJ2-PC peritoneal macrophage cell line. Unlike its parent compound with low water solubility, intraperitoneally administered ZWC is readily absorbed with no local residue or adverse tissue reaction at the injection site. Whether administered at or prior to the LPS challenge, ZWC more than doubles the animals' median survival time. ZWC appears to protect the LPS-challenged organisms by forming a complex with LPS and thus attenuating pro-inflammatory signaling pathways. These findings suggest that ZWC have utility as a systemic anti-LPS agent.

N-Lipidated Peptide Dimers: Effective Antibacterial Agents against Gram-Negative Pathogens through Lipopolysaccharide Permeabilization

Treating infections caused by multidrug-resistant Gram-negative pathogens is challenging, and there is concern regarding the toxicity of the most effective antimicrobials for Gram-negative pathogens. We hypothesized that conjugating a fatty acid moiety onto a peptide dimer could maximize the interaction with lipopolysaccharide (LPS) and facilitate the permeabilization of the LPS barrier, thereby improving potency against Gram-negative pathogens. We systematically designed a series of N-lipidated peptide dimers that are active against Gram-negative bacteria, including carbapenem-resistant Enterobacteriaceae (CRE). The optimized lipid length was 6-10 carbons. At these lipid lengths, the N-lipidated peptide dimers exhibited strong LPS permeabilization. Compound 23 exhibited synergy with select antibiotics in most of the combinations tested. 23 and 32 also displayed rapid bactericidal activity. Importantly, 23 and 32 were nonhemolytic at 10 mg/mL, with no cellular or in vivo toxicity. These characteristics suggest that these compounds can overcome the limitations of current Gram-negative-targeted antimicrobials such as polymyxin B.

Drug Carriers: Not an Innocent Delivery Man

Biomaterials used as drug carriers are often considered inactive and assumed to have no other roles than modifying pharmacokinetics and biodistribution of a drug. On the other hand, there are several examples in which the carrier materials show bioactivities in the body, which may have been underestimated or inadvertently ignored. This review highlights several examples where biomaterials used as drug carriers bring biological effects, known or newly discovered, and discusses their implications in development of new drug delivery systems.

Therapeutic interventions in sepsis: current and anticipated pharmacological agents

Sepsis is a clinical syndrome characterized by a multisystem response to a pathogenic assault due to underlying infection that involves a combination of interconnected biochemical, cellular and organ-organ interactive networks. After the withdrawal of recombinant human-activated protein C (rAPC), researchers and physicians have continued to search for new therapeutic approaches and targets against sepsis, effective in both hypo- and hyperinflammatory states. Currently, statins are being evaluated as a viable option in clinical trials. Many agents that have shown favourable results in experimental sepsis are not clinically effective or have not been clinically evaluated. Apart from developing new therapeutic molecules, there is great scope for for developing a variety of drug delivery strategies, such as nanoparticulate carriers and phospholipid-based systems. These nanoparticulate carriers neutralize intracorporeal LPS as well as deliver therapeutic agents to targeted tissues and subcellular locations. Here, we review and critically discuss the present status and new experimental and clinical approaches for therapeutic intervention in sepsis.

Marine compounds with therapeutic potential in gram-negative sepsis

This paper concerns the potential use of compounds, including lipid A, chitosan, and carrageenan, from marine sources as agents for treating endotoxemic complications from Gram-negative infections, such as sepsis and endotoxic shock. Lipid A, which can be isolated from various species of marine bacteria, is a potential antagonist of bacterial endotoxins (lipopolysaccharide (LPSs)). Chitosan is a widespread marine polysaccharide that is derived from chitin, the major component of crustacean shells. The potential of chitosan as an LPS-binding and endotoxin-neutralizing agent is also examined in this paper, including a discussion on the generation of hydrophobic chitosan derivatives to increase the binding affinity of chitosan to LPS. In addition, the ability of carrageenan, which is the polysaccharide of red alga, to decrease the toxicity of LPS is discussed. We also review data obtained using animal models that demonstrate the potency of carrageenan and chitosan as antiendotoxin agents.

Bacterial cell wall compounds as promising targets of antimicrobial agents I. Antimicrobial peptides and lipopolyamines

The first barrier that an antimicrobial agent must overcome when interacting with its target is the microbial cell wall. In the case of Gram-negative bacteria, additional to the cytoplasmic membrane and the peptidoglycan layer, an outer membrane (OM) is the outermost barrier. The OM has an asymmetric distribution of the lipids with phospholipids and lipopolysaccharide (LPS) located in the inner and outer leaflets, respectively. In contrast, Gram-positive bacteria lack OM and possess a much thicker peptidoglycan layer compared to their Gram-negative counterparts. An additional class of amphiphiles exists in Gram-positives, the lipoteichoic acids (LTA), which may represent important structural components. These long molecules cross-bridge the entire cell envelope with their lipid component inserting into the outer leaflet of the cytoplasmic membrane and the teichoic acid portion penetrating into the peptidoglycan layer. Furthermore, both classes of bacteria have other important amphiphiles, such as lipoproteins, whose importance has become evident only recently. It is not known yet whether any of these amphiphilic components are able to stimulate the immune system under physiological conditions as constituents of intact bacteria. However, all of them have a very high pro-inflammatory activity when released from the cell. Such a release may take place through the interaction with the immune system, or with antibiotics (particularly with those targeting cell wall components), or simply by the bacterial division. Therefore, a given antimicrobial agent must ideally have a double character, namely, it must overcome the bacterial cell wall barrier, without inducing the liberation of the pro-inflammatory amphiphiles. Here, new data are presented which describe the development and use of membrane-active antimicrobial agents, in particular antimicrobial peptides (AMPs) and lipopolyamines. In this way, essential progress was achieved, in particular with respect to the inhibition of deleterious consequences of bacterial infections such as severe sepsis and septic shock.

Exploring the LPS/TLR4 signal pathway with small molecules

The identification of the bacterial endotoxin receptors for innate immunity, most notably TLR4 (Toll-like receptor 4), has sparked great interest in therapeutic manipulation of the innate immune system. In the present mini-review, several natural and synthetic molecules that modulate the TLR4-mediated LPS (lipopolysaccharide) signalling in animals and humans are considered, and their mechanisms of action are discussed. The process of LPS sensing and signal amplification in humans is based on the sequential action of specific receptors situated in the extracellular side of the innate immunity cells, which bind and transfer LPS to TLR4: LBP (LPS-binding protein), CD14, MD-2 (myeloid differentiation protein 2). We classified the compounds active on TLR4 pathway depending on the specific molecular targets (LPS, LBP, CD14, MD-2 or TLR4). Small molecules developed by our group are described that inhibit LPS-stimulated TLR4 activation by selectively targeting the LPS-CD14 interaction. These compounds have an interesting antiseptic shock, anti-inflammatory and anti-neuropathic pain activity in vivo.

Development of small-molecule endotoxin sequestering agents

Sepsis, otherwise referred to as "blood poisoning" is a serious clinical problem, the incidence of which continues to rise in the US and worldwide despite advances in antimicrobial chemotherapy. The primary trigger in Gram-negative sepsis is endotoxin, a lipopolysaccharide (LPS) constituent of the outer membrane of all Gram-negative bacteria. The structurally highly conserved glycolipid called lipid A is the active moiety of LPS. Lipid A is composed of a hydrophilic, bis-phosphorylated di-glucosamine backbone, and a hydrophobic polyacyl domain. The bis-anionic, amphiphilic nature of lipid A enables it to interact with a variety of cationic hydrophobic ligands, including polymyxin B, a toxic peptide antibiotic which binds to lipid A and neutralizes endotoxicity. Having determined the structural basis of the interaction of polymyxin B with lipid A, our long-term goal has been to rationally design non-peptidic, nontoxic, small-molecule LPS-sequestrants. Our efforts began with defining the central pharmacophore that determined LPS-recognition and -neutralization properties in small molecules, which led to the discovery of a novel lipopolyamine lead, DS-96. DS-96 is an effective LPS-neutralizer, rivaling polymyxin B in a panel of vitro assays, as well as in protecting animals against endotoxicosis. Structure-activity relationships in our effort to rationally design endotoxin sequestering agents, preclinical assessment of hits and leads, and approaches to overcoming issues with toxicity are described in this chapter.

Glycolipids and benzylammonium lipids as novel antisepsis agents: synthesis and biological characterization

New glycolipids and a benzylammonium lipid were rationally designed by varying the chemical structure of a D-glucose-derived hit compound active as lipid A antagonist. We report the synthesis of these compounds, their in vitro activity as lipid A antagonists on HEK cells, and the capacity to inhibit LPS-induced septic shock in vivo. The lack of toxicity and the good in vivo activity suggest the use of some compounds of the panel as hits for antisepsis drug development.

Modeling the LPS neutralization activity of anti-endotoxins

Bacterial lipopolysaccharides (LPS), also known as endotoxins, are major structural components of the outer membrane of Gram-negative bacteria that serve as a barrier and protective shield between them and their surrounding environment. LPS is considered to be a major virulence factor as it strongly stimulates the secretion of pro-inflammatory cytokines which mediate the host immune response and culminating in septic shock. Quantitative structure-activity relationship studies of the LPS neutralization activities of anti-endotoxins were performed using charge and quantum chemical descriptors. Artificial neural network implementing the back-propagation algorithm was selected for the multivariate analysis. The predicted activities from leave-one-out cross-validation were well correlated with the experimental values as observed from the correlation coefficient and root mean square error of 0.930 and 0.162, respectively. Similarly, the external testing set also yielded good predictivity with correlation coefficient and root mean square error of 0.983 and 0.130. The model holds great potential for the rational design of novel and robust compounds with enhanced neutralization activity.

Structure-activity relationships of lipopolysaccharide sequestration in N-alkylpolyamines

We have previously shown that simple N-acyl or N-alkyl polyamines bind to and sequester Gram-negative bacterial lipopolysaccharide, affording protection against lethality in animal models of endotoxicosis. Several iterative design-and-test cycles of SAR studies, including high-throughput screens, had converged on compounds with polyamine scaffolds which have been investigated extensively with reference to the number, position, and length of acyl or alkyl appendages. However, the polyamine backbone itself had not been explored sufficiently, and it was not known if incremental variations on the polymethylene spacing would affect LPS-binding and neutralization properties. We have now systematically explored the relationship between variously elongated spermidine [NH(2)-(CH(2))(3)-NH-(CH(2))(4)-NH(2)] and norspermidine [NH(2)-(CH(2))(3)-NH-(CH(2))(3)-NH(2)] backbones, with the N-alkyl group being held constant at C(16) in order to examine if changing the spacing between the inner secondary amines may yield additional SAR information. We find that the norspermine-type compounds consistently showed higher activity compared to corresponding spermine homologues.

Pharmacokinetics of DS-96, an alkylpolyamine lipopolysaccharide sequestrant, in rodents

The pharmacokinetics of DS-96, an N-alkylhomospermine analog designed to sequester bacterial lipopolysaccharides, has been determined in rodent species. The elimination half-life in mice and rats are about 400 and 500 min, respectively, with other PK parameters being quite similar in the two rodent species. Interestingly, the mouse intravenous plasma concentration time curves exhibit an apparent absorption phase. While the rat intravenous data did not exhibit a pronounced apparent absorption phase immediately following injection, plasma levels did increase between 10 and 30 min following an expected drop from time 0 to 5 min. The data are consistent with first-pass uptake, possibly by the lung, with back diffusion as a function of time. The observed C(max) values of 1.36 microg/mL in the mouse intraperitoneal model suggest that a plasma concentration of 0.5-1 microg/mL corresponds to complete protection for a 200 ng/animal dose of intraperitoneally administered LPS in the D-galactosamine-primed model of endotoxin-induced lethality.

Structure-activity relationships of lipopolysaccharide sequestration in guanylhydrazone-bearing lipopolyamines

The toxicity of gram-negative bacterial endotoxin (lipopolysaccharide, LPS) resides in its structurally highly conserved glycolipid component called lipid A. Our major goal has been to develop small-molecules that would sequester LPS by binding to the lipid A moiety, so that it could be useful for the prophylaxis or adjunctive therapy of gram-negative sepsis. We had previously identified in rapid-throughput screens several guanylhydrazones as potent LPS binders. We were desirous of examining if the presence of the guanylhydrazone (rather than an amine) functionality would afford greater LPS sequestration potency. In evaluating a congeneric set of guanylhydrazone analogues, we find that C(16) alkyl substitution is optimal in the N-alkylguanylhydrazone series; a homospermine analogue with the terminal amine N-alkylated with a C(16) chain with the other terminus of the molecule bearing an unsubstituted guanylhydrazone moiety is marginally more active, suggesting very slight, if any, steric effects. Neither C(16) analogue is significantly more active than the N-C(16)-alkyl or N-C(16)-acyl compounds that we had characterized earlier, indicating that basicity of the phosphate-recognizing cationic group, is not a determinant of LPS sequestration activity.

Alexidine and chlorhexidine bind to lipopolysaccharide and lipoteichoic acid and prevent cell activation by antibiotics

OBJECTIVES

Many antibiotics used to treat infections cause release of immunostimulatory cell wall components from bacteria. Therefore, a combination of antimicrobial and endotoxin-neutralizing activity is desired to prevent inflammation induced by destroyed bacteria. Chlorhexidine and alexidine are amphipathic bisbiguanides and could neutralize bacterial membrane components as stimulators of Toll-like receptors (TLRs).

METHODS

Binding of chlorhexidine and alexidine to lipopolysaccharide (LPS) and lipoteichoic acid (LTA) was determined by fluorescence displacement assay and isothermal calorimetric titration. Neutralization of the biological effect of LPS and LTA on TLR-activated cellular activation was determined by NF-kappaB reporter luciferase activation on cells transfected with specific TLRs and NO production of murine macrophages in the presence of isolated agonists and antibiotic-treated bacteria.

RESULTS

Alexidine and chlorhexidine bind not only to LPS but also to LTA from Gram-positive bacteria. Alexidine has a higher affinity than chlorhexidine for both compounds. Calorimetric titration shows an initial endothermic contribution indicating participation of hydrophobic interactions in LPS binding, while binding to LTA displayed initial exothermic contribution. Both compounds prevent cell activation of TLR4 and TLR2 by LPS and LTA, respectively. The addition of both compounds suppressed NO production by macrophages in the presence of bacteria treated with different types of antibiotics.

CONCLUSIONS

Chlorhexidine and alexidine suppress bacterial membrane-induced cell activation at concentrations two orders of magnitude lower than that used in topical applications. Combining biocides with different types of antibiotics prevented macrophage activation in the presence of bacteria and demonstrated the potential of chlorhexidine and alexidine to suppress inflammatory responses caused by activation of TLRs.

Cationic liposomal lipids: from gene carriers to cell signaling

Cationic lipids are positively charged amphiphilic molecules which, for most of them, form positively charged liposomes, sometimes in combination with a neutral helper lipid. Such liposomes are mainly used as efficient DNA, RNA or protein carriers for gene therapy or immunization trials. Over the past decade, significant progress has been made in the understanding of the cellular pathways and mechanisms involved in lipoplex-mediated gene transfection but the interaction of cationic lipids with cell components and the consequences of such an interaction on cell physiology remains poorly described. The data reported in the present review provide evidence that cationic lipids are not just carriers for molecular delivery into cells but do modify cellular pathways and stimulate immune or anti-inflammatory responses. Considering the wide number of cationic lipids currently available and the variety of cellular components that could be involved, it is likely that only a few cationic lipid-dependent functions have been identified so far.

Association of glycemic profiles with whole blood polyamine among middle-aged Japanese men: colorimetric assay using oat and barley seedling polyamine oxidase

OBJECTIVES

Polyamines have long been known to have an insulin-like action, but their antiglycating effect has only recently attracted the attention of researchers. The aim of our investigation was to determine the whole blood polyamine concentration in a healthy population in order to examine its relationship with glycemic profiles.

METHODS

The study cohort comprised 622 men aged 40-59 who participated in a health checkup program conducted in 1997, when they underwent measurements of fasting plasma glucose (FPG), insulin (FPI), and fructosamine as glycemic indices. Colorimetric assay methods using oat and barley seedling polyamine oxidase were used to determine total polyamine (spermidine + spermine) and spermine concentrations in the whole blood, respectively. Polyamine concentrations adjusted for hemoglobin were quartiled for the analysis of covariance to assess the association with glycemic indices.

RESULTS

A significant association was demonstrated between the FPG and total polyamine concentrations. In the trend test, FPG and fructosamine levels increased in accordance with the shift of quartiles of total polyamine concentrations from low to high. In contrast, the association between the spermine and glycemic indices was not statistically significant based on the test for difference of multivariate-adjusted means or trend for linearity.

CONCLUSIONS

This is the first epidemiological study to reveal that the concentrations of blood polyamines are related with either FPG or fructosamine level in a healthy population. There may be some feedback mechanism for the elevation of circulating polyamines to quench the glycation reaction under hyperglycemic conditions. In addition, total polyamines, rather than spermine alone, seem to be a sensitive biomarker representing the antiglycation effect of polyamines.

Protection from endotoxic shock by EVK-203, a novel alkylpolyamine sequestrant of lipopolysaccharide

Lipopolysaccharides (LPS) play a key role in the pathogenesis of septic shock, a major cause of mortality in the critically ill patient. The only therapeutic option aimed at limiting downstream systemic inflammatory processes by targeting lipopolysaccharide is Toraymyxin, an extracorporeal hemoperfusion device using solid phase-immobilized polymyxin B (PMB). While PMB is known to effectively sequester LPS, its severe systemic toxicity proscribes its parenteral use, and hemoperfusion may not be feasible in patients in shock. In our continuing efforts to develop small-molecule mimics which display the LPS-sequestering properties, but not the toxicity of PMB, a series of mono- and bis-substituted dialkylpolyamines were synthesized and evaluated. We show that EVK-203, an alkylpolyamine compound, specifically binds to and neutralizes the activity of LPS, and affords complete protection in a murine model of endotoxic shock. EVK-203 is without any apparent toxicity when administered to mice at multiples of therapeutic doses for several days. The specific endotoxin-sequestering property along with a very favorable therapeutic index renders this compound an ideal candidate for preclinical development.

Polycationic Sulfonamides for the Sequestration of Endotoxin

Lipopolysaccharides (LPS) play a key role in the pathogenesis of septic shock, a major cause of mortality in the critically ill patient. We had previously shown that monoacylated polyamine compounds specifically bind to and neutralize the activity of LPS with high in vitro potency and afford complete protection in a murine model of endotoxic shock. Fatty acid amides of polyamines may be rapidly cleared from systemic circulation due to their susceptibility to nonspecific serum amidases and, thus, would be predicted to have a short duration of action. In a systematic effort to increase the likelihood of better bioavailability properties together with structural modifications that may result in gains in activity, we now report structure-activity relationships pertaining to endotoxin-binding and -neutralizing activities of homologated polyamine sulfonamides.

Structural correlation between lipophilicity and lipopolysaccharide-sequestering activity in spermine-sulfonamide analogs

Lipopolysaccharides (LPS), otherwise termed 'endotoxins', are outer-membrane constituents of Gram-negative bacteria, and play a key role in the pathogenesis of 'Septic Shock', a major cause of mortality in the critically ill patient. We had previously defined the pharmacophore necessary for small molecules to specifically bind and neutralize this complex carbohydrate. A series of aryl and aliphatic spermine-sulfonamide analogs were synthesized and tested in a series of binding and cell-based assays in order to probe the effect of lipophilicity on sequestration ability. A strong correlation was indeed found, supporting the hypothesis that endotoxin-neutralizing ability involves a lipophilic or membrane attachment event. The research discussed herein may be useful for the design of additional carbohydrate recognizing molecules and endotoxin-neutralizing drugs.

Structural correlates of antibacterial and membrane-permeabilizing activities in acylpolyamines

A homologous series of mono- and bis-acyl polyamines with varying acyl chain lengths originally synthesized for the purpose of sequestering lipopolysaccharide were evaluated for antimicrobial activity to test the hypothesis that these bis-cationic amphipathic compounds may also bind to and permeabilize intact gram-negative bacterial membranes. Some compounds were found to possess significant antimicrobial activity, mediated via permeabilization of bacterial membranes. Structure-activity relationship studies revealed a strong dependence of the acyl chain length on antimicrobial potency and permeabilization activity. Homologated spermine, bis-acylated with C8 or C9 chains, was found to profoundly sensitize Escherichia coli to hydrophobic antibiotics such as rifampin. Nonspecific cytotoxicity is a potential drawback of these membranophilic compounds. However, the surface activity of these cationic amphipaths is strongly attenuated under physiological conditions via binding to serum albumin. Significant antibacterial activity is still retained in the presence of physiological concentrations of human serum albumin, suggesting that these compounds may serve as leads in the development of novel adjuncts to conventional antimicrobial chemotherapy.

Novel endotoxin-sequestering compounds with terephthalaldehyde-bis-guanylhydrazone scaffolds

We have shown that lipopolyamines bind to the lipid A moiety of lipopolysaccharide, a constituent of Gram-negative bacterial membranes, and neutralize its toxicity in animal models of endotoxic shock. In an effort to identify non-polyamine scaffolds with similar endotoxin-recognizing features, we had observed an unusually high frequency of hits containing guanylhydrazone scaffolds in high-throughput screens. We now describe the syntheses and preliminary structure-activity relationships in a homologous series of bis-guanylhydrazone compounds decorated with hydrophobic functionalities. These first-generation compounds bind and neutralize lipopolysaccharide with a potency comparable to that of polymyxin B, a peptide antibiotic known to sequester LPS.

Molecular modeling analysis of the interaction of novel bis-cationic ligands with the lipid A moiety of lipopolysaccharide

Lipopolysaccharides (LPS), otherwise termed "endotoxins", are outer-membrane constituents of Gram-negative bacteria and play a key role in the pathogenesis of "septic shock", a major cause of mortality in the critically ill patient. We have shown that the pharmacophore necessary for optimal recognition and neutralization of LPS by small molecules requires an interaction between two protonatable positive charges separated by a distance of approximately 14A, which corresponds to the distance between two anionic phosphates on the glycolipid component of LPS called lipid A. The in silico binding of a diverse set of compounds with bis-amino, -amidino, -guanidino, and -aminoguanidino functionalities, identified as potential lead scaffolds in a high-throughput screen, with lipid A was explored using molecular docking simulations. A weighted expression for binding affinity was trained relative to experimental ED(50) measurements, attaining a correlation of R(2)=0.66. Our docking results showed that the electrostatic interaction between ligands and lipid A phosphates dominates the expression and varies little across the series, and other ligand-receptor interactions seem to play a secondary role in governing the observed variations in the relative ligand binding affinity. Further, it appears that the ligand internal energy plays the primary role in differentiating between compound binding affinities which also correlated well with experimental ED(50) data (R=0.77). Application of this strategy would be useful in the de novo design of highly active endotoxin-sequestering agents.

Forming and immunological properties of some lipopolysaccharide–chitosan complexes

The complex formation of lipopolysaccharide (LPS) with chitosan (Ch) was demonstrated using sedimentation velocity analysis in the analytical ultracentrifuge, centrifugation in glycerol gradient and isopicnic centrifugation in cesium chloride. An addition of Ch to the Escherichia coli and Yersinia pseudotuberculosis LPS solutions was found to result in formation of the stable LPS-Ch complexes. The interaction is a complicated process and depends on time and reaction temperature, as well as on the molecular weight of chitosan. A stable LPS-Ch complex could be formed only after preliminary incubation of the initial components at an elevated temperature (37 degrees C). It should be noted that process of LPS complexation with Ch is accompanied by additional dissociating of LPS. The complex formation was shown to be a result not only of ionic binding, but also of other types of interactions. The interaction of Ch with LPS was shown to modulate significantly the biological activity of LPS. The LPS-Ch complex (1:5 w/w) was shown to possess much lower toxicity in a comparison with the parent LPS at injection to mice in the similar concentration. The LPS-Ch complex was shown to maintain an ability to induce of IL-8 and TNF, but induction of IL-8 and TNF biosynthesis by the LPS-Ch complex was lower than that by the parent LPS. The complex LPS-Ch, similarly to the parent LPS, was found stimulated the formation of the IL-8 in the dose-dependent manner in the human embryonal kidney cells (HEK 293 cells) transfected with TLR4 in combination with MD2.

Structure−Activity Relationships in Lipopolysaccharide Neutralizers: Design, Synthesis, and Biological Evaluation of a 540-Membered Amphipathic Bisamide Library

Lipopolysaccharides (LPS), also called "endotoxins", are outer-membrane constituents of Gram-negative bacteria. Lipopolysaccharides play a key role in the pathogenesis of "septic shock", a major cause of mortality in the critically ill patient. We had earlier shown that small molecules bind and neutralize LPS if they contain (i) two protonatable cationic groups separated by a distance of approximately 14 A to facilitate interactions with the phosphate moieties on the lipid Angstrom component of LPS and (ii) a long-chain aliphatic hydrocarbon to promote hydrophobic interactions. In an effort to identify optimal scaffolds possessing the above structural requirements, we now present an evaluation of a rationally designed combinatorial library in which the elements of the scaffold are systematically varied to maximize sampling of chemical space. Leads obtained via molecular analyses of the screening results were resynthesized and evaluated in greater detail with regard to the affinity of the interaction with LPS, as well as neutralization of endotoxicity in in vitro assays. The examination of a moderately sized 6 x 6 x 15 (540-membered) focused library allowed the assessment of the structural contributions to binding by the long-chain aliphatic tails, distance between charged amino groups, and potential aromatic CH-pi or OH-pi interactions. These findings are of value in further iterations of design and development of specific and potent endotoxin sequestrants.

Lysine-spermine conjugates: hydrophobic polyamine amides as potent lipopolysaccharide sequestrants

Lipopolysaccharides (LPS), otherwise termed 'endotoxins', are outer-membrane constituents of Gram-negative bacteria. Lipopolysaccharides play a key role in the pathogenesis of 'Septic Shock', a major cause of mortality in the critically ill patient. Therapeutic options aimed at limiting downstream systemic inflammatory processes by targeting lipopolysaccharide do not exist at the present time. We have defined the pharmacophore necessary for small molecules to specifically bind and neutralize LPS and, using animal models of sepsis, have shown that the sequestration of circulatory LPS by small molecules is a therapeutically viable strategy. In this paper, the interactions of a focused library of lysine-spermine conjugates with lipopolysaccharide (LPS) have been characterized. Lysine-spermine conjugates with the epsilon-amino terminus of the lysinyl moiety derivatized with long-chain aliphatic hydrophobic substituents in acyl or alkyl linkage bind and neutralize bacterial lipopolysaccharides, and may be of use in the prevention or treatment of endotoxic shock states.

Combination of antimicrobial and endotoxin-neutralizing activities of novel oleoylamines

A combination of antimicrobial and endotoxin-neutralizing activities is desired in order to prevent progression from infection to sepsis due to the release of lipopolysaccharide from dying gram-negative bacteria. Lipopolyamines have emerged as a new type of endotoxin-neutralizing compound, but their antimicrobial activity has not been investigated. We synthesized a series of 10 oleoylamines differing in the polyamino head group, particularly in the number and separation between nitrogen atoms and the position of the oleoyl moiety. Compounds showed activity against both gram-negative and gram-positive bacteria in the micromolar range. Compounds were able to provide penetration of ethidium bromide into bacteria, indicating effects on the bacterial membrane. Oleoylamines neutralized endotoxin in Limulus amoebocyte lysate assays and by neutralization of tumor necrosis factor alpha release in human blood. Comparison of biological activities of compounds identified structural properties responsible for antimicrobial activity, and quantitative structure-property relationship analysis provided a quantitative model for prediction of activity of oleoylamines.

Functionalized dendrimers as endotoxin sponges

Lipopolysaccharides (LPS), otherwise termed 'endotoxins', are outer-membrane constituents of Gram-negative bacteria, and play a key role in the pathogenesis of 'Septic Shock', a major cause of mortality in the critically ill patient. We had previously defined the pharmacophore necessary for small molecules to specifically bind and neutralize LPS and, using animal models of sepsis, have shown that the sequestration of circulatory LPS by small molecules is a therapeutically viable strategy. Polyamidoamine dendrimers, with the surface amines substoichiometrically derivatized with alkyl groups bind LPS with high affinity, neutralize LPS-induced inflammatory responses in vitro, and afford protection in a murine model of endotoxic shock. Dendrimers represent a new class of potentially useful compounds for the therapy of Gram-negative sepsis.

Lipopolysaccharide sequestrants: structural correlates of activity and toxicity in novel acylhomospermines

Lipopolysaccharides (LPS), otherwise termed "endotoxins", are outer membrane constituents of Gram-negative bacteria. Lipopolysaccharides play a key role in the pathogenesis of "septic shock", a major cause of mortality in the critically ill patient. Therapeutic options aimed at limiting downstream systemic inflammatory processes by targeting lipopolysaccharide do not exist at the present time. We have defined the pharmacophore necessary for small molecules to specifically bind and neutralize LPS and, using animal models of sepsis, have shown that the sequestration of circulatory LPS by small molecules is a therapeutically viable strategy. In this paper, the interactions of a series of acylated homologated spermine compounds with LPS have been characterized. The optimal acyl chain length for effective sequestration of LPS was identified to be C(16) for the monoacyl compounds. The most promising of these compounds, 4e, binds LPS with an ED(50) of 1.37 muM. Nitric oxide production in murine J774A.1 cells, as well as TNF-alpha in human blood, is inhibited in a dose-dependent manner by 4e at concentrations orders of magnitude lower than toxic doses. Administration of 4e to d-galactosamine-sensitized mice challenged with supralethal doses of LPS provided significant protection against lethality. Potent antiendotoxic activity, low toxicity, and ease of synthesis render this class of compounds candidate endotoxin-sequestering agents of potential significant therapeutic value.

San-Huang-Xie-Xin-Tang reduces lipopolysaccharides-induced hypotension and inflammatory mediators

San-Huang-Xie-Xin-Tang (SHXT) is a traditional Chinese medicinal formula containing Coptidis rhizoma, Scutellariae radix and Rhei rhizoma. The present study aimed to determine the preventive effects of standardized SHXT on lipopolysaccharides (LPS)-induced arterial hypotension, protein expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2), cytokines formation and prostaglandin E2 (PGE2) production. LPS-induced activation of iNOS has been recognized to increase cytokines and nitric oxide, some of them play predominant roles in sepsis. Intravenous injection of LPS (10 mg/kg) caused a marked decrease of the mean arterial pressure in normotensive rats. However, the LPS-induced arterial hypotension was inhibited by SHXT (0.01 and 0.03 g/kg), when it was given 30 min before LPS. Moreover, plasma level of cytokines and PGE2 were lowered by SHXT. In RAW 264.7 cells, SHXT (20-200 microg/ml) dose-dependently inhibited LPS (1 microg/ml)-induced iNOS and COX-2 expression, and it also significantly decreased LPS-induced cytokines in a dose-dependent manner. In conclusion, our data suggest that SHXT prevented LPS-induced arterial hypotension, which might be mediated through its inhibition activities on the expression of iNOS and COX-2, cytokines formation and PGE2 production. Therefore, its protection activity against LPS-induced arterial hypotension and inflammatory mediators release might be beneficial in the treatment of endotoxin shock and/or associated inflammation.

Polycationic lipids inhibit the pro-inflammatory response to LPS

Lipopolysaccharide (LPS) is a major component of the outer membrane of Gram-negative bacteria. As such, it signals monocytes, macrophages and neutrophils to up-regulate phagocytic functions and to release pro-inflammatory cytokines. Despite the established role of CD14 as the main LPS receptor, the precise nature of the LPS signalling complex and its compartmentalization remain unknown. Interactions of LPS with other cell surface molecules such as TLR-4 and MD-2, and its subsequent internalization are required for LPS signalling. Here, we show that the polycationic lipid LipoFectamine causes inhibition of the LPS-induced MAPK activation and lack of pro-inflammatory cytokine production, despite proper localization of CD14 within lipid rafts and massive LPS internalization. The ability of LipoFectamine to inhibit LPS induced pro-inflammatory responses may be due to uncoupling of CD14 from TLR-4/MD-2 in the LPS signalling complex of mouse macrophages/microglial cells, as suggested by inhibition of LPS-induced concomitant internalization of these surface molecules. Thus, LipoFectamine may be a useful tool to dissect the molecular interactions leading to LPS signalling, and identifies a potential therapeutic strategy for LPS clearance.

Anti-endotoxin agents. 2. Pilot high-throughput screening for novel lipopolysaccharide-recognizing motifs in small molecules

Lipopolysaccharides (LPS), otherwise termed 'endotoxins', are an integral part of the outer leaflet of the outer-membrane of Gram-negative bacteria. Lipopolysaccharides play a pivotal role in the pathogenesis of 'Septic Shock', a major cause of mortality in the critically ill patient, worldwide. The sequestration of circulatory endotoxin may be a viable therapeutic strategy for the prophylaxis and treatment of Gram-negative sepsis. We have earlier shown that the pharmacophore necessary for small molecules to bind LPS is simple, comprising of two protonatable cationic functions separated by about 15 A, permitting the simultaneous interaction with the negatively charged phosphates on lipid A, the toxically active center of endotoxin. In this report, we employ high-throughput screening methods, using a novel fluorescent probe displacement method. Searches in three-dimensional structure databases yielded about approximately 4000 commercially available small molecules, each possessing two cationic functions spaced approximately 15 A apart. Approximately 400 such compounds have been screened in an effort to validate the method by which high-affinity endotoxin binders can be identified. We show that the IC50 values that are obtained from the fluorescence-based primary screen are correlated both to the enthalpy of binding, as measured by isothermal titration calorimetry, as well as to biological potency in vitro assays. By performing rapid toxicity screens in tandem with the bioassays, lead compounds of interest can be easily identified for further systematic structural modifications and SAR studies.